This invention relates to an improvement in a storage battery separator composed chiefly of a mass of glass fibers and a mass of fibril-formed synthetic fibers mixed therewith. Since glass fiber is rich in hydrophilic property at its surface and can be formed into very fine fiber, the same has been hitherto used for making a sealed-type lead storage battery separator in which the degree of liquid absorption is considered as one of the important characteristics. However, the glass fiber is smooth at its surface and is comparatively stiff, so that a sheet made of the glass fiber alone is inadequate in that the same is low in mechanical strength and does not have good handling properties. Accordingly, there has been hitherto proposed a type of separator in which the glass fiber is mixed with synthetic resin serving as a binding agent, or another type of separator prepared by mixing the glass fiber with synthetic resin monofilament fiber. However, this approach is still inadequate in that the same suffers a remarkable decrease in liquid absorption, and also is defective in that intertwining between the monofilament fiber and the glass fiber is not good and the mechanical strength improving effect is small. If, in addition, the amount of the monofilament fiber is increased, the strength of the separator is improved but the liquid absorption property is lowered.